Field of Application
The present invention relates to an optical wavelength filters for operating on light which propagates through an optical fiber or an optical waveguide. In particular, the invention relates to a form of optical wavelength bandpass filter whereby one or more narrow bands of wavelengths of light are transmitted through the filter, these bands being within a range of reflection wavelengths of the filter.
There are various types of optical wavelength filter, for reflecting or transmitting light of specific wavelengths, such as for example filters which utilize multi-layer reflective films, filters which use optical waveguide arrays, filters which use optical fiber gratings, etc. Of these, the optical fiber grating type of filter is most easily matched to use with an optical fiber as a transmission path, by comparison with other types of optical wavelength filters.
A type of optical fiber grating is known which is manufactured by using ultraviolet light to modify the diffraction coefficient within a section of the core of an optical fiber, such that the diffraction coefficient periodically varies along that section with a specific pitch. A corresponding narrow-band reflection spectrum is thereby obtained. If such a filter is directly inserted into a light path, then a narrow-band type of band rejection filter is obtained. However if such an arrangement is to be used to configure an optical wavelength filter, which is required to have one or more channels (i.e., respective bands of wavelengths which are passed by the filter, within a range of reflection wavelengths) then as shown in FIG. 8, it has been necessary in the prior art to use an additional element, i.e., an optical circulator 101. The light which is propagated through the optical fiber first passes through the optical circulator 101, to fall upon the optical fiber grating 102. Light of a specific range of wavelengths is reflected, while the remaining light is passed unchanged through the optical fiber grating 102. The reflected light is returned to the optical circulator 101, and passes out from the exit port 104, as emitted light 105. In that way, by using the emitted light 105 as the output light, the arrangement of FIG. 8 can be used as an optical wavelength bandpass filter which utilizes an optical fiber grating.
By using an optical fiber grating that is formed with a plurality of periodically varying diffraction coefficient regions, having respectively different values of period of variation of the diffraction coefficient, such a technique enables a multi-channel bandpass optical wavelength filter (or a multi-channel band rejection optical wavelength filter) to be obtained.
As another aspect of the prior art, it is known that a xe2x80x9cchirpedxe2x80x9d type of optical fiber grating can be used to obtain a wide-band reflection spectrum. With such a type of optical wavelength filter, the period of variation of the diffraction coefficient is continuously changed within the region of periodic variation of diffraction coefficient. That period of variation constitutes the grating pitch of the optical fiber grating. By altering the manner in which the grating pitch changes, an arbitrary reflection spectrum can be obtained. The chirped type of optical fiber grating is also a reflection type of device as described for prior art types of optical wavelength filter hereinabove. Hence, such a device is inserted directly in a light transmission path, to provide a band rejection optical filter. Thus, in the same way as described above, it is necessary to combine such a device with an optical circulator if it is required to implement a bandpass optical wavelength filter.
Hence, although an optical wavelength filter using an optical fiber grating has the advantage of compatibility with optical fibers, such a filter has the disadvantage that when used as a bandpass filter, it becomes necessary also use an optical circulator. By comparison with an optical fiber grating, an optical circulator-is an expensive item to manufacture. This problem of increased cost is therefore an obstacle to using such a type of filter, in addition to the disadvantage of having to use one additional element.
It is an objective of the present invention to overcome the disadvantages of the prior art set out above, by providing an optical wavelength filter using an optical fiber grating, whereby an optical wavelength bandpass filter can be realized without the need to use an optical circulator, so that a low-cost single-element bandpass filter can be achieved, for use in applications such as selection of optical communication channels.
It is a further objective to provide a simple method of manufacture for such a filter.
It is a further objective to provide a dynamically controllable type of optical wavelength filter, whereby the bandpass characteristics of the filter can be directly controlled by a user, rather than being fixedly determined at the stage of manufacture.
To achieve the above objectives, the invention provides a bandpass optical wavelength filter whereby the core or the cladding (or both the core and the cladding) of an optical fiber or an optical waveguide is formed with a periodically varying diffraction coefficient structure, with one or a combination of parameters of that structure continuously varying between opposing ends of that structure, such as to define a range of wavelengths of light which are reflected by the filter (referred to in the following as the reflection range), while in at least one location within the periodically varying diffraction coefficient structure a portion is formed (referred to in the following as an interruption portion) in which an interruption of the continuous variation of said parameter or parameter combination occurs. As a result, a corresponding narrow band of transmission wavelengths of the filter lying within the reflection range, i.e., a passband, is created. By using this technique, a single-element optical wavelength bandpass filter having one or more narrow passbands can be formed.
The invention discloses various ways in which a periodically varying diffraction coefficient structure having such interruption portions formed therein can be implemented. According to a first aspect of the invention, the pitch of the periodically varying diffraction coefficient structure, i.e., the grating pitch, is configured to continuously vary along that structure, and each passband portion consists of an interruption region within that structure, having a specific length, extending along the direction of propagation of light through the optical fiber or optical waveguide. Within an interruption region, no change in the pitch occurs. As a result, for each such interruption region, a specific range of wavelengths of light, within a reflection range, will be transmitted through the optical waveguide or optical fiber, with that transmission range of wavelengths being determined by the respective values of pitch immediately prior to the start of that interruption region and immediately following the end of the region.
According to a second aspect, the pitch is again continuously varied, and each of the passband portions consists of a discontinuity in the continuous variation of the pitch, defined at a specific position along the periodically varying diffraction coefficient structure. For each of these discontinuities, there is a corresponding range of wavelengths of light which will be passed by the optical waveguide or optical fiber, with that range determined by the respective values of pitch which occur immediately before and immediately following the discontinuity.
According to a third aspect, the periodically varying diffraction coefficient structure is formed with a fixed pitch, and the average value of diffraction coefficient within the periodically varying diffraction coefficient structure continuously varies along the direction of propagation of light through the optical waveguide or optical fiber. In that case, each of the passband portions consists of a discontinuity in the continuous variation of the average value of diffraction coefficient. For each of these discontinuities there is a corresponding range of light wavelengths which will be passed through the optical waveguide or optical fiber, i.e., a passband which is determined by the respective average values of diffraction coefficient which occur immediately before and immediately following the position of discontinuity.
Alternatively, according to a fourth aspect, the periodically varying diffraction coefficient structure is formed with a fixed pitch and the average value of diffraction coefficient within the periodically varying diffraction coefficient structure continuously varies along the direction of propagation of light through the optical waveguide or optical fiber. However in this case, each of the passband portions consists of an interruption region having a specific length extending along the propagation direction, in which the continuous variation of the average value of diffraction coefficient is interrupted. For each of these interruption regions there is a corresponding range of light wavelengths which will be passed through the optical waveguide or optical fiber, i.e., a passband which is determined by the respective average values of diffraction coefficient which occur at the opposing ends of that interruption region.
According to a fifth aspect, the features of successively varying the pitch and also the average value of diffraction coefficient along the periodically varying diffraction coefficient structure are combined, and each of the interruption portions is configured as an interruption region, in which both the continuous variation of the pitch and also of the average value of diffraction coefficient are interrupted.
According to a sixth aspect, the configuration is as described for the fifth aspect, other than in that each of the interruption portions consists of a discontinuity, at which both the continuous variation of the pitch and also of the average value of diffraction coefficient are interrupted.
In addition, the invention provides a simple method of manufacturing an optical waveguide or optical fiber in which one or more of the above-mentioned interruption regions are formed. The method utilizes a phase mask, with ultraviolet light passing through the phase mask to reach the optical waveguide or optical fiber, thereby producing periodic modulation of the diffraction coefficient, and with masking means provided for blocking the ultraviolet light from being incident on the optical waveguide or optical fiber at each of the aforementioned interruption regions.
According to another aspect, the invention provides an optical wavelength bandpass filter whose filter characteristics can be dynamically adjusted by a user, through selective application of heat generate by an array of heater elements disposed on an optical fiber or optical waveguide, to thereby produce alterations of diffraction coefficient such as to obtain a desired configuration for a periodically varying diffraction coefficient structure having one or more interruption portions which are defined based on any of the various possible configurations of an optical wavelength filter according to the present invention as summarized above.
It should be understood that the term xe2x80x9ccontinuous variationxe2x80x9d of a parameter of the periodically varying diffraction coefficient structure (continuous variation of pitch, and/or of average value of diffraction coefficient), is used in the following description and in the appended claims to signify a continuous variation of the parameter with respect to distance along that structure, specifically, continuous increase in value of that parameter between one end of the structure and the opposing end of the structure.